Buildings are often times designed with a space between a drop ceiling and a structural floor from which the ceiling is suspended to serve as a return air plenum for elements of heating and cooling systems as well as serving as a convenient location for the installation of communications cables and other equipment, such as power cables. Alternatively, the building can employ raised floors used for cable routing and plenum space. Communications cables generally include voice communications, data and other types of signals for use in telephone, computer, control, alarm, and related systems, and it is not uncommon for these plenums and the cables therein to be continuous throughout the length and width of each floor, which can introduce safety hazards, both to the cables and the buildings.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum space, and especially if flammable material occupies the plenum, the fire can spread quickly throughout the entire floor of the building. The fire could travel along the length of cables which are installed in the plenum if the cables are not rated for plenum use, i.e., do not possess the requisite flame and smoke retardation characteristics. Also, smoke can be conveyed through the plenum to adjacent areas and to other floors with the possibility of smoke permeation throughout the entire building.
As the temperature in a non-plenum rated jacketed cable rises, charring of the jacket material begins. Afterwards, conductor insulation inside the jacket begins to decompose and char. If the charred jacket retains its integrity, it still functions to insulate the core; if not, however, it ruptures due either to expanding insulation char or to pressure of gases generated from the insulation, and as a consequence, exposes the virgin interior of the jacket and insulation to the flame and/or the elevated temperatures. The jacket and the insulation begin to pyrolize and emit more flammable gases. These gases ignite and, because of air drafts in the plenum, burn beyond the area of flame impingement, thereby propagating flame and generating smoke and toxic and corrosive gases.
Because of the possibility of flame spread and smoke evolution, as a general rule, the National Electrical Code (NEC) requires that power-limited cables in plenums be enclosed in metal conduits. However, the NEC permits certain exceptions to this requirement. For example, cables without metal conduits are permitted, provided that such cables are tested and approved by an independent testing agent, such as Underwriters Laboratories (UL), as having suitably low flame spread and smoke generating or producing characteristics. The flame spread and smoke production of cables are measured using the UL 910 standard test method for fire and smoke retardation characteristics of electrical and optical fiber cables used in air handling spaces, i.e., plenums.
Communication systems in the present day environment are of vital importance, and, as technology continues to become more sophisticated, such systems are required to transmit signals substantially error free at higher and higher bit rates. More particularly, it has become necessary to transmit data signals over considerable distances at high bit rates, such as megabits or gigabits per second, and to have substantially error free transmission. Thus, desirably, the medium over which these signals are transmitted must be capable of handling not only low frequency and voice signals, for example, but higher frequency data and video signals. In addition, one aspect of the transmission that must be overcome is crosstalk between pairs of commercially available cables. One of the most efficient and widely used signal transmission means which has both broadband capability and immunity from crosstalk interference is the well known coaxial cable.
The coaxial cable comprises a center conductor surrounded by an outer conductor spaced therefrom, with the space between the two conductors comprising a dielectric, which may be air but is, most often, a dielectric material such as foamed polyethylene. The coaxial cable transmits energy in the transverse electromagnetic (TEM) mode, and has a cut-off frequency of zero. In addition, it comprises a two-conductor transmission line having a wave impedance and propagation constant of an unbounded dielectric, and the phase velocity of the energy is equal to the velocity of light in an unbounded dielectric. The coaxial line has other advantages that make it particularly suited for efficient operation in the hf and vhf regions. It is a perfectly shielded line and has a minimum of radiation loss. It may be made with a braided outer conductor for increased flexibility and it is generally impervious to weather effects. Inasmuch as the line has little radiation loss, nearby metallic objects and electromagnetic energy sources have minimum effect on the line as the outer conductor serves as a shield for the inner conductor. As in the case of a two-wire line, power loss in a properly terminated coaxial line is the sum of the effective resistance loss along the length of the cable and the dielectric loss between the two conductors. Of the two losses, the resistance loss is the greater since it is largely due to skin effect and the loss will increase directly as the square root of the frequency.
The most commonly used coaxial cable is a flexible type having an outer conductor consisting of copper or aluminum wire braid, with the copper or aluminum inner conductor supported within the outer by means of the dielectric, such as foamed, or expanded, polyethylene (XPE), which has excellent low-loss characteristics. The outer conductor is protected by a jacket of a material suitable for the application, such as, for example, for non-plenum use, poly(vinyl chloride) (PVC) or polyethylene (PE).
The coaxial cable most preferred for its performance characteristics for non-plenum uses has an XPE dielectric and PVC jacket. However, the use of XPE dielectric material and a PVC jacket generally does not result in a cable that satisfies UL 910. The use of foamed perfluorinated ethylene polymers, such as polytetrafluoroethylene (PTFE) and perfluorinated ethylene-propylene polymer (FEP), both sold under the trademark TEFLON.RTM., has been suggested for the dielectric material due to its low flame spread and low smoke emission characteristics. However, foamed polyethylene is preferable because it is cheaper and requires simpler processing techniques. When accompanied with a plenum grade jacket, a cable having an XPE dielectric material will usually satisfy UL 910. TEFLON.RTM. is also useful as a plenum grade cable jacket material. However, TEFLON.RTM. is quite expensive and is currently in extremely short supply, hence is unsatisfactory from an economic standpoint, although outstanding for its flame and smoke retardation characteristics.
In general, highly flame retardant cable jackets have been made in two ways. An inert flame retardant additive such as antimony or molybdenum can be added to an appropriate polymer, such as PVC. Alternatively, or perhaps in combination, a halogenated polymer that is inherently flame retardant (such as TEFLON.RTM.) can be used alone or as a copolymer.
It is apparent from the foregoing discussion that what is still sought is an inexpensive, flame retardant, and low-smoke generating coaxial cable with excellent electrical transmission capabilities. The sought after cable desirably is easy to manufacture and does not sacrifice transmission properties for fire and smoke resistance.